Diffusion furnace with high speed recovery



May 28, 1968 G. P. HAMPTON Filed June 21, 1967 DIFFUSION FURNACE WITH HIGH SPEED RECOVERY BY 7%, W/O/ZMI/ Attorneys United States Patent Gmce 3,385,921 Patented May 28, 1968 ABSTRACT OF THE DISCLOSURE Diffusion furnace having a high speed recovery control system which is self-compensating for different sizes of boats by measuring the temperature of the boat.

Background of the invention This invention relates to diffusion furnaces with high speed recovery. A diffusion furnace utilizing high speed recovery has heretofore been provided of the type described in Patent No. 3,311,694. It has been found that this type of diffusion furnace is particularly useful but that it has the disadvantage in that it must be recalibrated for each different type of load which is inserted into the furnace.

Summary of the invention The diffusion furnace consists of means forming a processing zone which is adapted to receive a load to be processed. Heating means is disposed in the zone and means is provided for sensing the temperature of the heating means. Means is also provided for sensing the temperature of the load. A controller is provided for controlling the supply of energy to the heating means. Means is connected to the controller and to the means for sensing the temperature of the heating means and the means for sensing the temperature of the load to supply a signal to the controller which is proportional to the tempera ture of the heating means and the temperature of the load so that the load will be heated up to the set point for the controller without overshooting.

In general, it is an object of the present invention to provide a diffusion furnace with high speed recovery which is selfcompensating.

Another object of the invention is to provide a diffusion furnace of the above character in which the temperature of the load is measured.

Another object of the invention is to provide a diffusion furnace of the above character which is easy to adjust.

Another object of the invention is to provide a diffusion furnace of the above character in which side mounted or end mounted thermocouples can be used.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

Brief description of the drawings The drawing shows a partially schematic isometric illustration of a diffusion furnace incorporating the present invention with the associated control circuitry.

Description of preferred embodiment As shown in the drawing, the diffusion furnace consists of a furnace assembly 11 similar to that described in United States Letters Patent No. 3,311,694. As described therein and as partially shown in the drawing, the diffusion furnace also includes a control system 12. Each of the furnace assemblies 11 consists of a processing tube 14 formed of a suitable material, such as quartz, which serves as means for forming a processing zone 16 within the same which is adapted to receive a load to be processed.

Typically, the load can take the form of a boat 17 formed of a suitable material, such as quartz, and which is adapted to carry wafers 18 that are to be subjected to diffusion operations. The processing tube 14 is provided with an open end 21 through which the boat can be inserted and removed. In addition, it is provided with a neck 22 having an opening 23 therein through which gases utilized during the diffusion processes can be introduced.

Heating means is provided for supplying heat to the processing zone 16 in the processing tube 14 and consists of a helical heating element 26 which extends longitudinally of the processing tube. Means is provided for shielding the processing zone from radiant energy from the heating element 26 and consists of a tube 27 formed of a suitable material, such as mullite. The tube 27 extends the length of the processing tube 14. The tube 27, in addition to acting as a shield for radiant energy, serves as a heat sink.

The furnace assembly 11 also includes a rectangular casing (not shown) and a suitable insulating material, such as fire brick, which is disclosed in Patent No. 3,311,694 for enclosing the heating element 26 and the tube 27 and the processing tube 14 carried therein so that they are insulated from the atmosphere.

As shown in the drawing, the processing zone 16 is divided into three zones, zone A, zone B and zone C. The control system 12 includes means for supplying energy to the heating element 26 from a suitable source of power connected to lines L1 and L2 such as 208 volts AC. Transformers TRl, TR2 and TR3 are provided for each of the three zones and are connected to suitable power supplies identified as PS1, PS2 and PS3, respectively. The power supplies can be of a suitable type such as silicon controlled rectifier assemblies. As shown in the drawings, the power supplies PS1, PS2 and PS3 are all connected to the lines L1 and L2. The transformers TR1, TR2 and TR3 are connected to three portions of the heating element 26. Means is provided for controlling the power supplies PS1, PS2 and PS3. Such means can consist of null galvonometers G1, G2 and G3 or, alternatively, solid state control amplifiers, and a set point reference supply C-1 of the general type described in Patent No. 3,311,694. The reference supply C-1 includes a set point deviation meter 36 and supplies a reference voltage.

A thermocouple assembly consisting of thermocouples T1, T2, T3, T4 and T5 is provided similar to that described in US. Letters Patent No. 3,291,969 except they are isolated electrically from each other and are connected to the set reference supply C-l as indicated. The

thermocouples T1 and T4 are positioned in zones A and C, respectively, whereas the thermocouples T2 and T3 are positioned in zone B. Preferably, these thermocouples are positioned so that they can sense the temperature within the processing zone. The master thermocouple T5, on the other hand, is preferably positioned so that it can measure the temperature of the heating element 26 in the central zone B.

An additional thermocouple T6 is provided for sensing the temperature of the load. It is preferable that this thermocouple be as close to the load as possible but, for purposes of convenience, can be positioned adjacent to the load to measure the temperature of the load. Thus, as shown in the drawing, the thermocouple T6 can be mounted in a block 31 within the processing tube 14 in zone B and immediately underlying the boat 17. Alternatively, if desired, this thermocouple can be positioned in the processing or diffusion tube 14 adjacent the load. As can be seen from the drawing, the negative lead from the master thermocouple T5 and the thermocouple T6 are connected in common and are connected to the reference supply C-1. The positive terminal of. the master thermocouple T5 is connected to one side of a proportioning potentiometer P-l. The other side of the potentiometer is connected between two resistors R1 and R2 which are connected to opposite sides of a precision power supply 3-1 as, for example, a l-millivolt supply. The resistors R1 and R2 which are connected in series have their ends connected in parallel with an adjustment potentiometer P-Z. The wiper of the potentiometer P4 is connected to the positive terminal of the thermocouple T6. The wiper of the proportioning potentiometer P-l is connected to a self-return toggle switch S1 which is connected to another terminal of the reference supply C-l. In one position of the switch S1, it is connected to the wpier of the potentiometer P-1, whereas in the other position of the switch, it is connected to the side of the potentiometer oposite the side to which the positive terminal of the thermocouple T5 is connected.

Operation of the diflusion furnace may now be briefly described as follows. To initially calibrate thediffusion furnace, the potentiometer P1 is turned fully clockwise as viewed in the drawing. This means that the reference supply C-l would be receiving 100% of its signal from the master theremocouple T5 and the operation of the diffusion furnace would be substantially identical to that described in Patent No. 3,291,969. The diffusion furnace is permitted to reach equilibrium with the potentiometer P-l in this position. As soon as this has been accomplished, the self-return toggle switch S1 is shifted to the left to, in effect, connect the other side of the potentiometer P1 to the reference supply C-1. As soon as this has been done and while the switch S1 is held in this position, the potentiometer P4. is adjusted so that the deviation meter 36 of the reference supply C1 is agam at zero. Thereafter, the toggle switch S1 is released and the temperature measured by the master thermocouple T5 is again supplied to the controller. If the deviation meter is not at zero, then the toggle switch 51 should be quickly shifted back so that the other side of the potentiometer P-1 is connected to the reference supply C-1 and a final adjustment is made on the potentiometer P2. The toggle switch S1 is again released so that the other side of potentiometer P-1 is connected to the reference supply C1. This procedure is continued until the reference supply C-l can be switched from one side to the other of the potentiometer P-l. without a change on the deviation meter. This means that each of the two thermocouples provide the same voltage and are are an equilibrium condition that results in a constant voltage across the entire potentiometer P-l.

After the diffusion furnace has been calibrated, its manner of operation is as follows. Let it be assumed that the potentiometer P-l has been set so that it takes proportional amounts of the voltage from the master thermocouple T5 and from the thermocouple T6 as, for example, 90% from the thermocouple T5 and 10% from the thermocouple T6. Let it also be assumed that the diffusion furnace has reached equilibrium and that it is desired to insert a load in the diffusion furnace. As soon as the load represented by the boat 17 is introduced into the processing zone 16 of the processing tube 14, the thermocouple T6 senses the substantially lower temperature of the load and signals to the reference supply -1 that the diffusion furnace is below the desired temperature. Additional energy will be supplied to the heating element to raise the temperature of the heating element and to increase the temperature of the body within the diffusion zone. This increased temperature is sensed by the thermocouple T and is also supplied to the reference supply C-l. Thus, it can be seen that the potentiometer P-1 divides the controller input so that it has at least a portion thereof which is proportional to the heating element temperature and also to the temperature of the load.

Normally, it is the desire of this invention to bring the load up as quickly as possible to the optimum tern perature or, in other words, the set point provided by the reference supply C-l without overshooting. In order to avoid overshoot, it is desirable at a certain point in time that the temperature of the load increase at the same rate that the temperature of the heating element comes down so that there is no overshoot. In the event the temperature of the load overshoots the desired optimum temperature, the potentiometer P-1 is shifted so that proportionally less signal is received from the load and an increased signal is received from the heating element. Conversely, if the temperature of the load does not come up to the desired diffusion temperature within a relatively short time, the potentiometer P-1 is shifted so that a greater proportion of the signal supplied to the reference supply ()4 is supplied from the load. With this arrangement, it can be seen that it can be readily adjusted and that if overshoot is occurring, it is merely necessary to adjust the potentiometer P1 until there is no overshoot.

Once the control system has been adjusted, the system is self-compensating and does not need to be adjusted for each boat which is placed in the diffusion furnace because in the diffusion furnace, the temperature of the boat is being measured.

In the initial adjustment of the diffusion furnace with the potentiometer P-2 and with the voltage supply B1 of 1 millivolt, it can be seen that by adjustment of the potentiometer P-2, it is possible to add one-half of a millivolt in each direction. By way of example, if the thermocouple T5 measuring the temperature of the heating element is reading 1,000 C. and the thermocouple T6 measuring the temperature of the boat is reading 990 C., then by use of the potentiometer P2, sufficient voltage is added from the millivolt supply to the potentiometer that 10 would be added to the voltage supplied by the thermocouple T6 so that both sides of the potentiometer P-1 would have a voltage representing 1,000. When such is the case, the potentiometer P4 can be shifted from one end to the other without changing the furnace temperature.

The foregoing mode of operation describes the operation for the center zone B of the diffusion furnace. The end zones A and B are slaved to the center zone and operate in the same manner as described in Patent No. 3,291,969. It should be appreciated that, if desired, three separate thermocouples could be provided for measuring the temperature of the boat with one of the thermocouples being used for each of the zones to control the temperature of the zone in the same manner that the central zone B is controlled.

The block 31 serves as a thermal mass within the diffusion tube 14 and also serves to keep the boat 17 level within the tube. By way of example, the mass 31 could be formed of solid graphite covered with quartz. The block or slab 31 normally would have a length substantially equal to the length of the center zone B. The thermal mass provided by the slab 31 would help to ensure that the entire boat 17 would be brought up to temperature uniformly.

It is apparent from the foregoing that there has been provided a new and improved diffustion furnace which has high speed recovery when a load is inserted in the diffusion furnace. In addition, it is self-compensating and need not be adjusted for all different sizes of boats which are placed in the diffusion furnace. It also can be readily adjusted for relatively great accuracy.

I claim:

1. In a diffusion furnaces, means forming a processing zone adapted to receive a cold load to be processed, heating means for supplying heat to the processing zone, means for sensing the temperature of the heating means, means for sensing the temperature of the load in the processing zone, control means connected to said means for sensing the temperature of the heating means and to said means for sensing the temperature of the load, said control means having adjustable means for adjusting the output from the control means so that a predetermined portion thereof is proportional to the temperature of the heating element and a predetermined portion thereof is proportional to the temperature of the load to bring the load up as quickly as possible to the desired temperature without overshoot.

2. A furnace as in claim 1 wherein said diffusion furnace is formed with first, second and third zones and wherein said load is disposed in said second zone and wherein said control means includes means for slaving said first and third zones to said second zone.

3. A diffusion furnace as in claim 1 wherein said control means includes a reference supply, a voltage supply, a first potentiometer having one end connected to the voltage supply and having the other end connected to the means for sensing the temperature of the heating means, said first potentiometer having a wiper connected to the reference supply, and a second potentiometer having both ends connected to the voltage supply and having a wiper connected to the means for sensing the temperature of the load for providing a combined voltage output which is substantially identical to the voltage output from the means for sensing the temperature of the heating element.

4. A diifustion furnace as in claim 3 wherein said control means includes switch means movable between two positions, said switch means in one of the positions being connected to the wiper of the first potentiometer and in the second position being connected to said one end of the first potentiometer.

5. A diffustion furnace as in claim 1 wherein a block is disposed in the diffusion zones and said load is carried by said block and wherein said means for sensing the temperature of the load senses the temperature of the block.

References Cited UNITED STATES PATENTS 3,291,969 10/1966 Speransky et a1. 2l9497 XR 3,227,797 1/1966 Rees 2l9494 XR ROBERT K. SCHAEFER, Primary Examiner.

M. GINSBURG, Assistant Examiner. 

